Emulation Mode Flag: Difference between revisions

Revision as of 13:57, 24 July 2024

The Emulation Mode Flag (E) controls whether the 65c816 is behaving like a 6502.

When clear, the MPU is in 65c816 native mode.

When set, the MPU is in 6502 emulation mode.

This flag cannot be modified directly and is normally hidden from the programmer. To modify it, use the XCE instruction to swap its value with the carry flag's value. This treats the status register bits as a game of musical chairs.

The flag is set when a RESET interrupt is fired. In other words, the CPU always starts in emulation mode at boot and after a reset.

The 65c816 does not attempt to emulate the illegal 6502 opcodes.

For writing new SNES code it is recommended to always stay in native mode even if dealing with 8 bit data. One reason is page boundary crossing incur a one cycle penalty in emulation mode. Emulation mode emulates the NMOS 6502 cycle counts.

Emulation code need not be in bank 0.

References

Figure 17.3, Eyes & Lichty page 377, https://archive.org/details/0893037893ProgrammingThe65816/page/377
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#APPENDIX

@@ Line 5: / Line 5: @@
 When set, the MPU is in '''6502 emulation mode.'''
-This flag cannot be modified directly and is normally hidden from the programmer.  To modify it, use the [[XCE]] instruction to swap its value with the [[carry flag]]'s value.
+This flag cannot be modified directly and is normally hidden from the programmer.  To modify it, use the [[XCE]] instruction to swap its value with the [[carry flag]]'s value.  This treats the status register bits as a game of musical chairs.
 The flag is set when a [[RESET]] interrupt is fired. In other words, the CPU always starts in emulation mode at boot and after a reset.